1. Field of the Invention
The present invention relates to a synchronous machine control apparatus equipped with a power converter that rotary drives a synchronous machine.
2. Description of the Related Art
Synchronous machines include a permanent magnet synchronous motor having a permanent magnet in a rotor and a reluctance motor that generates a torque by utilizing magnetic saliency of a rotor. When such a synchronous machine is controlled by a synchronous machine control apparatus having a power converter, such as an inverter, control widely performed in the related art is to orient an armature current vector in a certain phase direction with respect to the rotor. For example, in the case of a permanent magnet synchronous machine in the related art, an absolute value of an armature current vector is controlled in proportion to a desired torque by controlling the armature current vector to be oriented in a direction orthogonal to a permanent magnet flux axis of the rotor. Meanwhile, in the case of the reluctance motor, it is known that an absolute value of the armature current vector is not proportional to an output torque and it is therefore difficult to perform torque control with a high degree of accuracy by the control method in the related art.
Further, when a rotation speed of the permanent magnet synchronous motor increases, an inductive voltage induced by a permanent magnet flux causes an armature voltage to rise and the armature voltage eventually exceeds a voltage that can be outputted from a power converter, such as an inverter. In order to prevent such an inconvenience, flux weakening control is performed to reduce an armature interlinkage flux by generating a negative armature current vector called a weakening current in a direction of the permanent magnet flux axis. It should be noted, however, that an armature voltage varies with an output torque even when the weakening current is the same. It is therefore difficult to control an armature voltage to have a desired value according to the magnitude of a torque by the control method in the related art.
An example of a synchronous machine control apparatus configured to solve this problem is disclosed, for example, in U.S. Pat. No. 7,554,281 (Patent Document 1). The disclosed synchronous machine control apparatus includes: a torque current computing unit that computes a torque current command that is a torque component of an armature current command on the basis of a torque command and a flux command; a torque current limit generator that generates a maximum torque current command value that can be generated on the basis of a magnetizing current command that is a magnetizing component of the armature current command and a current limit value of a power converter in such a manner that the armature current does not exceed the current limit value; a torque current command generator that is formed of three components forming a limiter that imposes a limitation on the torque current command according to the maximum torque current command value; a flux command generator that computes a flux command according to the torque current command from the torque current command generator; a flux computing unit that computes an armature interlinkage flux on the basis of an armature current of the synchronous machine alone or the armature current and an armature voltage; and a flux controller that generates a magnetizing current command so that the flux command and the armature interlinkage flux are consistent with each other and inputs the magnetizing current command into the torque current command generator.
According to the configuration above, by calculating the torque current command with reference to the flux command and the magnetizing current command, it becomes possible to take the limit of an output current of the power converter into consideration, and also by calculating the flux command with reference to the torque current command, it becomes possible to generate a suitable flux command that reflects a fluctuation of the torque current command caused by limiting the output current as described above.
Another example of the similar control apparatus is disclosed, for example, in Japanese Patent No. 3640120 (Patent Document 2). This control apparatus generates an armature interlinkage flux command instructing the synchronous machine to generate desired torque and armature voltage and an armature current (torque current) command orthogonal to the armature interlinkage flux command. Meanwhile, the control apparatus determines a current (magnetizing current) command in a flux axis direction so that an armature interlinkage flux found by a flux computation on the basis of the armature current becomes consistent with the armature interlinkage flux command. Thereafter, the control apparatus generates a current command of a rotary biaxial coordinate (hereinafter, referred to as the d-q axes) rotating at an angular frequency on the basis of the torque current command and the magnetizing current command.
According to this configuration, it becomes possible to improve control characteristics by linearizing a relation between a torque and a current of the synchronous machine and to reduce a capacity of the power converter by directly controlling a terminal voltage.
Still another example of the similar control apparatus is disclosed, for example, in Japanese Patent No. 3570467 (Patent Document 3). This control apparatus includes a unit that computes, on the basis of a torque command value and an armature interlinkage flux command value, a current command value with which a torque and an armature interlinkage flux can be controlled linearly. When the torque command value is smaller than a predetermined value, the interlinkage flux command value is used as an increasing function of the torque command value whereas when the torque command value is greater than the predetermined value, the interlinkage flux command value is limited to a certain value equal to or less than an upper limit value.
According to this configuration, it becomes possible to improve control characteristics by linearizing a relation between a torque and a current of the synchronous machine and to avoid magnetic saturation caused by limiting the upper limit of the interlinkage flux.    Patent Document 1: U.S. Pat. No. 7,554,281 (FIG. 1 and the description thereof)    Patent Document 2: Japanese Patent No. 3640120 (FIG. 3 and the description thereof)    Patent Document 3: Japanese Patent No. 3570467 (FIG. 5 and the description thereof)
Patent Document 1 refers to the configuration to output a flux command with reference to the torque current command in such a manner that a torque reaches a maximum value under the condition that magnitude of a current is constant. However, a condition that allows a torque to reach a maximum value under the condition that magnitude of a current is always constant is not the same as a condition that allows conversion efficiency (conversion efficiency of the synchronous machine or the power converter alone or total conversion efficiency of the synchronous machine and the power converter) to reach a maximum level. In use with which there is a need to obtain a larger torque from a smaller current, it is desirable to set an operation target so that control is performed to allow a torque to reach a maximum value under the condition that magnitude of a current is constant. However, the control in this manner is not necessarily desirable all the time.
In cases where a temperature rise of the synchronous machine becomes a problem, for example, where a cooling performance is poor or a synchronous machine has a permanent magnet synchronous machine inside, desirable control is to allow conversion efficiency of the synchronous machine to reach a maximum level so that heat generation in the synchronous machine is suppressed, or where there is a need to consider heat resistance of a switching device forming the power converter of the synchronous machine, desirable control is to suppress heat generation in the power converter. It is therefore difficult for the control device of Patent Document 1 to flexibly address these cases.
The control apparatuses of Patent Documents 2 and 3 can directly control a torque current and an armature interlinkage flux with a high degree of accuracy in consideration of magnetic saturation and a maximum output voltage across the power converter. However, the torque current command and the flux command are generated to allow a torque to reach a maximum value under the condition that magnitude of a current is constant as in Patent Document 1. Hence, there is the same problem as that in Patent Document 1.
An operation target, such as the maximum conversion efficiency, varies from time to time with conditions (chiefly, temperatures) of the synchronous machine and the power converter. It is therefore necessary to provide a configuration that generates an operation target command that suits a situation online.